Right now it's just a pot controlling the height, but I was thinking over the weekend that it wouldn't be that hard to put 4 photoresistors at equal distances along the tube to serve as a control mechanism (light would reflect better off the white ball).
I was thinking of using a IR analog sensor ( http://www.hobbyengineering.com/H1060.html ) mounted on the face of the fan looking up at the ball. I have been looking for a nice useless project so that I could work with a PID control library and I think your project looks to be as useless as they come. ;D
Lefty